First of all I would like to point out that I do NOT think the Tiger is a slow boat. I am still racing one and feel that I am competitive with the rest of the fleet.
Having said these are things that I have done but do not promote doing:
Shortening the bridle wires and lenthening the forestay. I have pulled the jib down to where the newer Tigers jib will be. I did this know that I am putting more sideways pressure on the hulls but feel that the boat is built well enough to take it. In doing this mod I am using a bridle forestay connector from Nacra.
I have the newer rudders on the boat which I really like. Some think it is not a good idea. I feel that the boat sails more agile. It used to be harder to pull the bows down without stalling the rudders. By the way these rudders put the leading edge further forward of the pivot point so they have a power steering feel to them.
Other things that cannot be attributed to actual boatspeed but I would argue that convenience equates to speed:
Endpole spinnaker - I am the only one that likes it so do not go out and change your boat immediately. I feel that we can hold the spinnaker up longer at the leeward mark with this set up. Others think it is too much windage.
Mast Rotation adjusters on the hull for ease.
16 to 1 downhaul
Spinnaker halyard cleat up on the mast so Eileen can stand while hoisting.
Footstraps at the very back of the boat so crew can feel safe trapping downwind.
Spinnaker is neither old style or new style but somewhere in between. I think the new style is designed for speed. Problem is that it is hard to hold and makes the crew tired quickly, thus not always trimmed. The old style was easy to hold but not as fast. I have a Glaser sail which is easier to hold than the new cut and I feel just as fast. If my spinnaker works optimally all the time and yours, due to fatigue, works only 80% of the time, I am faster.
Double ratcheting blocks on the spinnaker sheet for all of the same reasons as the last paragraph. One of them I can turn off in light winds.
Center strap on the trampoline so I can sit comfortably downwind. Because the crew is all the way back while trapped out downwind I have to move in a bit so the spin sheet can clear my body. I am all the way back and do not think I am hindered much in this position. I can more easily see under the main at traffic, VERY IMPORTANT! By the way I also send Eileen out on the wire early so she can hold the spin sheet easily.
I make my own harnesses. I have not found one that is more comfortable that the ones I make. I have done this for close to 20 years. Another advantage to this is if I need to modify them I am not afraid to since I did not spend $300 each on them.
Oh I almost forgot. Keep the deck layout clean and the boat in good shape. Change rigging. Do not keep the same stuff more than two years.
Last but not least. SAIL A LOT! You do not have to be racing to learn how to go fast. Just having the boat under you as much as you can will make you faster.