I wouldn't be too sure. The low-end 13" MacBook Air has a 1.7Ghz dual-core i5 processor, 4GB RAM, a 128GB SSD, and a Thunderbolt port providing two bidirectional 10Gb/s channels.
From what I can find, an IBM S/390 from the early 1990s could have up to 6 processors and 6 vector coprocessors, up to 9GB of internal storage, and up to 256 fiber-optic links running at 10MB/s (for an total bandwidth of a Thunderbolt port plus a USB 2.0 port).
I don't really know how fast the CPU and RAM were in the IBM mainframe, but I'd suspect the Air's clock speeds are high enough to make up for only having two processors, and the Air's SSD would make it much faster for data sets that don't fit in RAM.
And as for the reliability advantage a mainframe is supposed to have: you could buy several MBAs per month to act as hot-spares for the price of renting the mainframe, and the MBA has a UPS built-in.
It's easy to forget that 1Gbit/second an insane amount of text / numberic data and 1990 was a LONG time ago. Just concider that the extimated monthly transfter acroos internet backbones in December 1990 was 1TB. That 1GBit lan cable was 100's of times faster than all of the internet backbones in 1990.
So while, Mainframe software is vary efficent the hardware still sucked compared to modern systems. Just think a 1GBit/second Fible Channel did not show up until 1997 http://en.wikipedia.org/wiki/Fibre_Channel.
PS: The #5 super computer in June 1993 had 4 cores Processor NEC 400 MHz (6.4 GFlops). http://www.top500.org/system/377 A 999$ mackbook air uses a 1.6GHz dual-core Intel Core i5 processor that would crush it in large part due to that 3 MB L3 cache but also due to being able to do far more in of those cycles.
An interesting aspect is how much more slowely memory sizes have caught up. Today's laptops can crush a 90s supercomputer in GFlops, but are just now catching up to a mid-1990s Cray in main memory (4GB). I remember being fairly befuddled when I started college in 2000, and my CS dept's UltraSPARC servers were slower than my Pentium III in CPU, but had memory measured in a unit that I had only heard of being used for hard drive space.
A fair comparison though would have to be a modern Cray-class supercomputery thing to that Cray though. That's going to have more than 4GB of RAM; modern Cray equivalents open the bid at petabytes of RAM.
Oh, I agree modern supercomputers beat 1995-era ones on all axes; it's mostly the different rates at which consumer-class computers catch up that are interesting. A modern MacBook Pro wipes the floor, CPU-wise, with a 1995 Cray, but only just matches it RAM-wise. So comparisons like "today's [consumer thing] is as good as [year's] [enterprise thing]" depend heavily on whether you're comparing them on CPU or RAM.