It makes it 340282366920938463463374607431768211456 times easier to brute force.
So if you could guess 10^9 per second it'd only take you 10^24 years instead of 10^63 years.
256 bit ECDSA keys only have 128 bit security. Half of an ECDSA key would be 64 bit security. While a naive attack would be to increment all possible private keys there are more sophisticated attacks (
https://en.wikipedia.org/wiki/Pollard%27s_rho_algorithm_for_logarithms ) which are of complexity O( n^(1/2) ) steps where n is the key length.
64 bit security would be breakable but it is very likely the cost to break they key would be greater than the reward. Although if this isn't a hypothetical I would recommend transferring the coins now.
Thanks for answers.
Reason I asked is that I prefer to give half a key to Alice and Bob and the other half to Charlie and Dennis. I consider this a backup strategy in case something happens to me (so my family can inherit) or I lose my notes (e.g. house burns down). To add safety I could use multisig, but the risks to worry about should be elsewhere; may these conspire and steal my coins, did a screen capture see the private key, did I write them down without any typos?
Why don't you copy/paste 1/2 of your private key separately and put each half in a safe deposit box. You could set up each box so that while you are alive, only you have access to both boxes, but upon receipt of notification that you have died, Bob would have access to one box and Allice would have access to the other.
This issue with multi sig is that both parties could potentially work together to gain access to your funds, as well as the fact that multi sig is relatively new to the Bitcoin network.
Writing down your key is not a good idea as even one "typo" would cause your entire "key" (it wouldn't actually be a key anymore) worthless