Hi,
Chulai.
I can't believe I forgot to mention the build I was using. My bad... Well, anyway, it was this one:
GoldenDict 1.0.1-239-gc81e6ca of March 3, 2012. Following your advice and switching to a newer build
GoldenDict 1.0.1-271-geaa62e6 of July 19, 2012 didn't make Thai dictionaries perform any better.
If I try searching any of those dictionaries for a word starting with a particularly common letter so that the number of probable hits is initially high, I have to wait up to 10-15 seconds before I'm able to type in the next character while the CPU usage spikes to 50-60%. There's clearly a bug somewhere...
And while we're on the subject, I thought I'd mention this one:

&

.
Don't know if you guys know of it yet. It's there in both of those builds.
Also, totally off topic now, but I was wondering if you could tell me whether it's possible to somehow associate language groups with keyboard layouts and tabs with language groups, and if not, whether there're any plans afoot to make this possible?..
PS. As to LSD vs. DSL, obviously I was using DSL files. I did mention, didn't I, that I was using GoldenDict under Windows XP. The list of links was only meant as an example of the dictionaries I was talking about.