I find just like you say unless I go really really hard at the beginning my HR also takes about (hard to say exactly but...) 10 minutes to get to zone 5a/5b which is approximatly 89 - 90% of max HR. I believe that is why on the 30 minute itt they say take the HR for the second part (20 minutes). Also it seems that after 10 minutes your HR finally settles down, mine right at 170 - 171, 87 percent of max.
Now if I really want to get my HR up fast I do a fast sprint start and I can hold my HR at 178 - 180 BPM (92 - 95%) for as long as I can suffer through it, usually about 8 - 10 minutes at race pace there. After about 10 minutes fatague starts seriously setting in at that point and I come close to blowing up. The longest in a itt I have held 92 - 95% max is during a 11k tt, and I believe that was 18 - 20 minutes or so but you would have to scrape me off the ground after.
To be honest I pay less attention to HR on the shorter itts than on the longer ones, and more to percieved exsertion. I went out and preran Saturday's 4.1 miles (6.2k), a very very hilly route in 12minutes 5 seconds and ran a 90 percent HR, but I admit I was not motivated to go 95%, I am estimating mid 11's on Saturday.
Hope that helps...
