For a first speed run, I suggest trying an least amount of in-game days to completing the main quest.
Some tips:
1) No Town Hall Quests. Looking for rumors just eats up a lot of in-game time.
2) Avoid unnecessary side quests. Sure, you'll have to knock out some places as part of the level-up process, but there's a ton of sidequests and story arcs that can be skipped.
3) Class: Try the Cleric in the Warrior Guild (Might 1, Heal 3, Heavy Hand 3, Intervention 1, Wisdom) Pretty basic build that can deal damage, can heal, and find those tough secret doors at an early level.
4) Companion: Get Gris when you can, switch to Hirge. Use her as a healer (Heal only active). Having two Clerics turns a lot of mana into HP which gives you a lot of playing time between inn rests.
5) No houses. Using the gold for potions equipment will help you push faster through the game.
6) You need 12 shards. Doesn't matter which ones. Some are a lot easier to get. Go for them.