I joined the industry about the same time as you and, over the years have written more assembler and COBOL than I care to remember, though in the last decade my C output has just about caught it up. In summary:
My first computer was an Elliott 503 that I learnt Algol 60 on before graduating.
In 1968 I joined ICL and spent my next two years writing PLAN 3 assembler before learning COBOL. Then it was George 1, mini-MOP, George 3 and programming in COBOL, JEAN, PLAN, the PLAN macro-generator and Algol68R.
Since then its been ICL 2903, ICL 2900 and no more professional assembler. Then on to Stratus, Tandem NonStop, DEC VAX, S/38, S/88, AS/400 in COBOL, TAL, PL/1, RPGIII, C running under VME/B, VOS, Guardian, VMS, OS/400, Unix.
Home stuff:
MC 6809, Flex 09, lots of assembler, PL/9, BASIC and C
MC 68020, OS-9, C, Sculptor 4GL, some assembler
Intel X86 - no more assembler, but lots of C and then Linux, C and Java
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Re: the IBM 5150 and speed
The IBM office automation systems of the era (System/3, System/34 and System/36) were notoriously slow. They did all arithmetic using variable length packed BCD values. I remember hearing rumours at the time that IBM had mandated that the PC must not be faster than an entry level S/36 in case it affected S/36 sales.
Shortly after the 8 MHz PC-AT was launched a third party S/36 emulator, the Baby/36, appeared. It emulated a S/36 with one terminal and would run all the standard S/36 software including the development tools. IBM sales guys hated Baby/36 because, on a PC-AT, it was quite a bit faster than an entry level S/36 and a fraction of the price, but S/36 developers loved it.
Contemporary non-IBM clones were generally faster than IBM PCs. IBM salesmen, dyed in the wool mainframers to a man, really put the PC down as a piece of insignificant garbage. So, do I believe that IBM PCs were slow by design? Definitely!