The Macintosh Performa 600 (aka Macintosh IIvi) was a bit of a crippled machine. They reused many aged components from older machines, most notably the memory and SCSI controllers. Also, if your 600 is running a Quantum ProDrive LPS, you would not be experiencing the pinnacle of speed. Those drives, though interesting bits of engineering and design, were VERY slow. I use them because, well, they're interesting bits of engineering and design, plus a Classic can't handle much in the way of fast drives anyway. Also, flaky drives, poor/improper termination, and otherwise poor connections or cables would also affect performance. I had a Performa 450 with a bad hard drive (didn't know it was bad at the time), and it took anywhere from three minutes to an hour to boot, if it finished the process. A new drive cured it.
The system bus difference shouldn't matter too much, really, but I'm afraid that I've never calculated the throughput of either machine, so I can't accurately say.
As for your screen problems, depending on your comfort level in dealing with sensitive and fragile components (now more fragile and sensitive due to advanced age), you could always reseat some cables on the LCD panel itself. The variable resistors used for the contrast/brightness controllers tend to develop carbon tracks that impair functionality, resulting in touchy or non-functional switches. They may be cleanable with some isopropyl alcohol, but it would be best to probably replace those if they're really bad. It would also be a good idea to look for oxidized solder joints on any exposed components on the display as well. Many 140s I've seen had oxidized solder joints on some resistors that caused the display to quit functioning, but reworking resulted in a nice, crisp display... er, as crisp as a 1-bit STN display can be, anyway.
The system bus difference shouldn't matter too much, really, but I'm afraid that I've never calculated the throughput of either machine, so I can't accurately say.
As for your screen problems, depending on your comfort level in dealing with sensitive and fragile components (now more fragile and sensitive due to advanced age), you could always reseat some cables on the LCD panel itself. The variable resistors used for the contrast/brightness controllers tend to develop carbon tracks that impair functionality, resulting in touchy or non-functional switches. They may be cleanable with some isopropyl alcohol, but it would be best to probably replace those if they're really bad. It would also be a good idea to look for oxidized solder joints on any exposed components on the display as well. Many 140s I've seen had oxidized solder joints on some resistors that caused the display to quit functioning, but reworking resulted in a nice, crisp display... er, as crisp as a 1-bit STN display can be, anyway.