After I migrated a Windows XP computer from SeaMonkey 1.1.19 to SeaMonkey 2.0.6, it turned out that the program experienced short, yet periodic slowdowns which were also evident in Process Explorer with CPU spikes that were related to the process.
The particular profile of SeaMonkey has the following extensions:
Flashblock 1.3.16, NoScript 2.0.1 and three spelling dictionaries.
As I surfed around to look for any kind of resolution, I stumbled upon the two most relevant forum threads, one in MozillaZine and the other on NoScript.
The best hint was given in this InformAction posting, where the workaround is to disable ABE (Application Boundaries Enforcer), which also disabled the holdups, but I hope for a better solution without sacrificing some on security.
Where to disable ABE:
NoScript Options > Advanced tab > ABE subtab > Remove checkbox at "Enable ABE (Application Boundaries Enforcer)"