Assuming you are already using an epoxy with a slow hardener (e.g MAS), sometimes when it is hot I will keep the resin in the refrigerator, removing it just before mixing each batch.  By the time the cold resin is thoroughly mixed with hardener that is at room temperature. the epoxy a little viscous but still spreadable.   This approach works best if you weigh your epoxy and hardener instead of using pumps.   In fact I don't know why anyone fiddles with pumps when weighing is much more accurate and much quicker.  Digital scales are very inexpensive these days.  I also don't use a roller for the initial wet out.  I find it much much faster to dump a puddle of resin on top of the glass and spread it out with a spreader.  Finally, maybe try to find a partner to mix batches while you are spreading so that you can maintain a wet edge as you work from one end of the boat to the other

Thanks for the suggestion about refrigeration. Yes, I am using the slow hardener. The boat is already glassed - I started it at a CLC class, but there is a lot more surface area that is now bare wood. I guess that in itself doesn't make a difference, but I think I would find it difficult to pour and spread over the inverted hull or along the inside gunwales. I have learned my lesson about bristle brushes. No matter how much I work at pulling the loose bristles out before starting, I always end up with hairs in the epoxy. I have a box of foam brushes now ready for use. 

   Sorry, I did not realize that the outside of the hull was already glassed.  In that case a roller is indeed the best option

I am curious about your recommendation to weigh instead of pump. What size batches are you typically measuring out?

   I am in the process of building a 15' sailboat (First Mate designed by Ross Lillestone).   For the wet out I used batches of 250 grams of MAS LV resin which requires 113 grams of slow hardener (the ratio by weight is slightly less than the 2:1 ratio by volume).  I never actually looked at the volume, but the quart cups I used seem slightly less than half full.   Room was about 70 degrees but epoxy had been left outside overnight and was about 55 degrees.   The entire wet out took about 90 minutes.  After mixing a batch I immediately poured all of it onto the bottom of the hull and then used a spreader to distribute it along the bottom and sides.  As resin starts to run down the sides I used the spreader to distribute it longitudinally.  Yes, stuff drips on the floor - sometimes a lot.  Only occasionally did I use a chip brush to help wet out the sides.  After wetting out each section, I returned to the previous section and used the spreader to remove any excess resin (this resin always gets discarded)..   By mixing (for exactly 3 minutes) and immediately dumping the resin there is almost no chance for heat build up in the mixing cup (or roller tray) so one has basically the cure time of the resin to work the epoxy.  For the fill coats I used 200 gram batches of resin with medium hardener and a roller.  

   I forgot to state that each batch gets a clean cup.  This insures that the cure times are not impacted by the heat generated by any residual resin in earlier batches.

Thanks for the additional detail.  I sure wish it would get down to 55 F overnight here - I live outside Houston, and it will be a while before we see that cool temperature.  I also make sure to use a new cup - I am sure that at least some of my non-curing attempts were because I reused a cup and had residue that interfered with the right mix of a new batch.

  Try using a West Systems with Extra Slow Hardener.

Try putting crazy glue on your TA brush at the base of the bristles.  This will help hold them in place.   Cut off the bristles to half-length also helps.