Lately, something hasn't been right with my rear shifter. When I shift up (to smaller cogs), there is sometimes a 3-5 second delay between me pushing the lever and the actual gear shift.

This problem does not occur when shifting down to lower gears (to larger cogs). Only when I shift up.

What's the problem and what should I do? I'm thinking of taking it in to the bike shop today.

The problem is likely to be that you need to adjust the tension on the gear cable. Do this with the barrel adjuster where the cable enters the rear dérailleur. If the lag is when you change up to a harder gear your need to turn the adjuster clockwise - not too much, proceed in quarter turns while turning the cranks and shifting the gears. It's easiest if you have a work stand, but you can do it perfectly well with the bike upside down.

If you are struggling to follow this explanation, simply google for " adjusting a rear dérailleur". There's a ton of videos on YouTube. It couldn't be easier, no need for a mechanic.

I doubt it is a cable adjustment causing this because OP reports it doesn't happen when shifting into a larger cog, only when going into a smaller one, and OP didn't mention continuous chain-skipping during the delay.

More than likely the problem is drag somewhere in the shifting mechanism. This probably just means that the cables and housing are gummed up and need to be cleaned and lubed. Also it could mean the cable is damaged or there is a kink in the cable where it enters housing and that is causing drag. Another possibility is very dirty cable guides under the BB.

The solution is to clean and lube the cables and housings, and the bb guide, and if they're worn or the cable is damaged, replace. There is probably a youtube video on this.

+1

Could be. Certainly worth a look, anyway, lots of people are riding around with corroded/dirty cables.

Unlubricated/dirty/rusted derailleur cable and housing, broken strands, or bent housing that are causing high friction.

Lots of people recommend replacing housings annually which is over-kill, but even 3000-4000 miles of relatively dry weather can be enough to break a few cable strands and for oil to dissipate. Someplace like Seattle probably calls for more regular maintenance.

The problem is caused by the friction that slows down the cable tension release. I would trace the RD cable from the shifter all the way to the RD and see if there is anything that may cause the increase in friction. One time I had the RD cable zip tied accidentally at the down tube and it had the same effect as you described.

Also, take a look at the bottom bracket cable guide underneath the bottom bracket shell, the plastic piece where your gear cables go through. It gets dirty easily.

This is almost certainly a cable friction issue that you can solve simply by getting new cables and housings. Do not shoot lube down your cable housings -- you'll just gum things up more.

But if you do lube make sure it is only in the housing. Exposed lubed cables will just catch dirt that may find its way into the housing and cause drag.

easy fix is to detension the cable and clean the dirt out of the loop of housing that enters the derailleur itself. those are traps for dirt. Mine exhibited that delay this am also. expect dirt in the housing at the derraileur end.

Thanks guys, I'll see what I can do with it myself.

So I checked things out myself, and the cables and housings didn't look that dirty. I was tempted to lube the cable inside the housing just to make sure, but I didn't want to gum it up even more. So I took it to the mechanic (he checked my bike before and he was willing to look at it for free). He tested the shifters and adjusted the cables a bit, but he said he didn't know what was wrong with it in the first place. I asked him if the cables needed tightening or loosening or if the housing needed to be cleaned, and he said it all looked fine. Just to try it again and if I still have the same problem, he'll try something else.

I'm going on a ride tonight, so we'll see.

If you can't remember when you changed the cables and housing just change them. Your shifting will improve. It's a cheap fix and a good idea to do on a regular basis.

The issue with the cables inside the housing is not dirt. The housings generally have some type of low friction lining which gets worn over time. You won't be able to see any signs of wear as it is internal to the housing.

Just change them.

Check the rear derailleur with your hand. Try moving jockey wheel sideways. Does it moove sideways like it has some "play". It happens on lower quality derailleurs - they get some play and shift slower. If rear derailleur has significant sideways freeplay, it could be the cause of slow shifting.

definitely check the bb cable guide for gunk. this will cause all sorts of drag and ghost shifting issues...