I think, its the custom tt's recently added, which you need a mappack for. I have even experienced SAMP Crashes in /TT due to these custom tts. Better gid rid of these maps.
Definetly not for this. To play mappack tracks you have to download it first, otherwise if you try to race them without it, nothing will happen and you'll just fall to the ground. The mappack it's not within the server (like MTA, for example) each player has to download it in order to player the races, it means that it's not a server-side option (The only process that the server does is load the CPs, like every other race) but a client-side option, so only those who are interested in racing the maps will install the mappack, the rest will just play with stock GTA. And if you're experiencing lag in /TT it's for sure not for those 10-15 races, just think a the amount of LSTestTrack races, they are something like 250 and nobody since the day they've been added encountered any lag when joining TT.