This is just my experience, as great as 8.6 is it does have a huge system overhead with or without extensions. Though not nearly as large as the overhead of 9, but... 601, 603 and some 604 based computers running 8.6 take forever starting up even with a moderate number of extensions and control panels. The previous posts advice is great, but take it one step further and time the startup with extensions off and then time it with extensions on. That gives you a diagnostic point to work from.
I would also suggest getting a copy of conflict catcher. Not that you have any conflicts, but it gives a lot more feedback during startup by identifying many, but not all, extensions and control panels that are being loaded on startup. It can also be a great tool to have around in case you do think that a conflict is causing you problems.