Here's how it works:
1) I sell some stock that's held at my bank. The bank takes me 1-2 days to get me cash from the sale.
2) I try to initiate a SWIFT transfer, and inevitably fail, because my bank sucks at SWIFT transfers.
3) I spend a couple days in a frustrating interaction with tech support until they finally get it to go through.
4) I sit around waiting for the cash to post.
As for "transferring 50% of your assets on each buy": not long ago, it was 100%. This is only my third transfer, so obviously it's going to be a large percentage of the total. It was a particularly large sale last time, and the value of my IB account is down due to TSLA being down, so... this is the result.
I'm not sure who or what you're talking about when you write "icelandic brokers don't leave cash in brokerage accounts". At my bank I have an account denominated in krónur (for my personal needs) and an account denominated in dollars (for my investments). I lose money on every dollar / króna conversion (a lot), so I avoid these conversions like the plague. E.g. technically it would be possible for me to sell stock (denominated in dollars), and then instead of waiting for it to post to my dollar-denominated account, I could "borrow from myself" from the króna-denominated account to initiate a SWIFT transfer. But then I'd be facing extra króna-dollar conversions.
You talk about whether the bank could use my króna-denominated account as collateral and do direct SWIFT transfers to IB for me. Um... maybe? But they seem to have trouble with anything even remotely complicated, so I doubt it. It was a multi-week process just trying to establish a way to do limit orders with them.
Regardless, it's all a moot point now (well, this time at least) because the transfer posted this morning.
Now to try to get a good price on the buy... annoying that the stock is surging premarket :Þ