A few years ago I did a no-reload speed run just for BG1, using SCS and various other mods. I had a number of attempts at that using different strategies and have copied my best effort below from the Bioware Forum. Building on that I attempted to complete the whole trilogy no-reload using a basic BGT installation on core difficulty in a single 24 hour period.
My rules are essentially that there's no cheating, e.g. no use of the console or keyboard cheats and no use of exploits such as teleporting through walls, creating duplicate items or casting spells via potions. However, what some people would define as cheese, e.g. running between areas to avoid combat, is allowed. Time spent starts from initial loading of the game and includes any need for breaks (or falling asleep) as well as game delays such as playing cut-scenes.
To reduce wasted time thinking during the game I developed a script to follow during a couple of trial run-throughs with a dwarf fighter/thief named Alac. There are separate posts below with the scripts for BG1, SoA, ToB and the intended level-up progression. Text in italics indicates items to be taken.
The approximate timings I've put next to encounters for SoA and ToB are based on a couple of test runs I did. They suggest that the game can be finished in something like 12-14 hours if things go well. I suspect that physical and mental fatigue will require play in the later stages to be somewhat slower in order to stay safe, but I would still expect to be able to finish in less than 24 hours if I don't fall asleep ...
I wanted to have a full day without other commitments and had a go in July 2014 - 2 attempts, the second of which ended when getting blocked in while trying to exit the Underdark. A further couple of attempts were made in May 2015 - the first ended by a bouncing lightning bolt from Mekrath and the second when I failed to pay attention to the damage being done by an incendiary cloud trap in Spellhold. Succeeding in the challenge is definitely realistic though and I'm expecting to have another go when I've got a full day free of commitments.
I had 4 more goes in October 2016. Three of those were relatively short-lived in one day - two of them ending in the Planar Prison and one when I tried to disarm the trap guarding the tome in Durlag's Tower (which can't be disarmed in my installation) and was charmed. The fourth attempt a couple of days later went almost all the way in just over 15 hours (including breaks), but failed on Mel's 3rd incarnation at the Throne when I misplaced spike traps waiting to ambush her so that she destroyed rather than triggered them with her arrival - and with Alac caught by surprise and unbuffed Mel went straight for a timestop and cut him to pieces. A brief summary of that run is on the no-reload thread at https://forums.beamdog.com/discussion/comment/817985/#Comment_817985