Worth noting w/ re 1400 vs. 2400/3400:
They are radically different architectures, with the 2400/3400 being one or two generations newer, on a much newer and better overall architecture. (this is even though they are basically contemporaries.)
The 1400, remember, is a mobile Performa/PMac 6200, whereas the 2400/3400 has a PCI architecture, closer to the Mac 6400.
MHz for MHz, the 1400 will always bench significantly worse than basically everything else. My own 1600/166 is handily outbenched by 120 (and probably 100, if I'm honest..) MHz PCI PowerMacs. (new thing to try on my 8500....) (this is all also true of the PowerBook 5300 and Duo2300). These are sometimes worse than the 6200/6300, because the 6200 at least also has 256k of L2 cache, and PowerBooks 5300/2300/1400 sometimes omitted it, or had only 128k of L2, depending. L1 is usually the 6300 "fixed" 32k of unified L1 vs. 8+8 split L1, so they at least don't have that part of the problem.)
One thing I didn't see addressed: What OS/software do you have on your 1400? Although these things can technically run 8.5 or 9.1, 7.6.1 or 8.1 might be a better compromise. You still get a lot more PPC code than 7.5 but they're significantly lighter-weight.
(though: disclaimer: my 1400's on a 30-gig IDE spinner, I haven't tried replacing it for SD or CF yet.)
One more thing: Getting a "good" SD card matters. These things "should" be able to turn in decent-for-the-time numbers. 6200 outbenches 6100 at disk... See if you can pick up a UHS-1 card or a card rated for sustained writes - that's where I had the best luck in my own SCSI2SD v6. (I know that's a different deployment scenario, but still.)
In terms of 1400s being weird with some storage: I've seen this cut both ways and I suspect it has, basically, to do with luck and all the exactly specific parts. Some people have reported no CF cards except for weird specific industrial ones work, and some people have reported varying luck with IDE<>SD adapters. Today, I'd say go to SD if you can since you can get bigger, faster cards and you'll be able to source them further into the future.
To both of those points... most CF cards, especially anything smaller than liek 4-8 or so gigs, wasn't meant in any way at all for computer use. These were meant for low speed, memory buffered, and/or read-oriented use cases. (although booting is read-oriented, booting a computer and, say, playing an MP3 are still different use cases, really.)