...all composite boats as it seems theraform may be slower due to a less rigid construction.
Thermoformed is about as rigid as composite, so won't really be any slower. It is a touch heavier (in general) than composite, which may be more of a concern for a smaller paddler. If there was a thermoformed boat that fit you, I wouldn't automatically discount it.
Rotomolded plastic is less rigid and even heavier. Truthfully, the speed difference on less rigid isn't something I think anyone except a finely tuned racer would notice. But that weight penalty off water you definitely will notice.
I'd like to stick to a longer (~17 ft) boat as my impression is they will keep up speed better and be more stable.
Waterline length mostly impacts hull speed, which is the maximum speed you can hold before it takes a huge jump in power to get above. The longer the water line the higher the hull speed. Moving from a 16' boat to an 18' boot maybe raises the hull speed by a half knot.
But hull speed is a sprinting speed, not a cruising speed. At cruising speeds, waterline length has little to no effect on drag. So waterline length really isn't that important for cruising speed. And this is good, as the boat length doesn't always match the waterline length. A highly rockered boat curves up at the end, so has a shorter waterline length. The Avocet LV you mentioned has a decent amount of rocker, so its waterline length is quite a bit less than its ~16' length.
Instead it is whetted surface and how much your boat has to split the water are the primary drivers of cruising speed. Keeping it simple, a narrower boat would be faster than a wider boat. And the more surface area of water hitting hull (whetted surface), the more drag.
A boat floats based on how much water it displaces All things equal, a lighter person on a fixed length boat could use a boat that is less wide. So you at 120 lbs in a 17' boat could take a boat that is almost half as wide (mathematically) than I can at 220 lbs. That narrower boat would in theory have less drag than mine (if you could find boats that had same design but just different widths to match our weights). But, that narrower boat would be markedly less stable than mine.
Many longer, expedition boats with a smaller paddler will paddle better with extra gear be added as ballast. If you do a majority of paddling on longer trips, having the added ballast is fine. But note that adding ballast also pushes the boat down further, increasing your whetted surface slowing you down some (though a small amount).
But if you are like most of us and 90+% of your trips are day trips, you may want to stick to a smaller boat that doesn't require ballast and just learn to pack lighter. This may give you the best tradeoff between packing capacity, speed, and balance.
Confuse you yet? Guess the long story is don't get set on any certain specification (like wanting 17' long) as that may not match what you really want (speed with capacity and decent balance). Everything is a trade off. You really need to just try boats and see what works for you.
Some thoughts on specific boats:
The Avocet LV composite is known as a good smaller person boat, but is know more as playful than speedy. Side note- plastic version of Avocet LV doesn't have the same feel and isn't as well regarded as a smaller person boat.
If you see a Valley Gemini, you might want to consider. They are known as good smaller person boats that are low rocker and hold more speed than expected out of a 15' boat. ST model would best match your needs, but RM and SP could also work for you. My 5'1" 110 lb girlfriend loves her RM.